I really don't get what you're saying. Even if immunity is "short-lived", by your own admission, it's not less than 3 months. So, for the 3 hottest months of the year, why should those with antibodies wear masks?
We can't be sure how long immunity to the COVID virus lasts. Here's one way we can find out: We take a group of people who were all infected around Purim, and once a month we inject them with coronavirus. We do this for 3 months, 4 months, 8 months, 2 years... as long as it takes to find someone who gets sick a second time. I've
posted earlier on the ethical problems of doing an experiment like this.
The alternative we have is to simply wait... and see how long it takes for re-infections to be reported. It might be never! It's possible that immunity is lifelong. But since we don't know, we have to continue to be careful, at last until a vaccine or a definitively effective treatment is developed.
The other alternative is to look for insight in the behavior of other coronaviruses. For other coronaviruses, immunity may last 3-12 months, at least in some individuals. From what we've seen so far, you are right: It looks like it
does last at least 8 months. But remember, many of the people who were initially infected 8 months ago in China are continuing to SD and wear masks. So the lack of reinfection may be partly due to continued immunity, and partly due to their protecting themselves from contact with the virus.
Re: the summer. There's no reason to think that summer temperatures will stop the virus. Covid was spreading in Florida during the winter/spring when temperatures were pleasantly warm. There have been peaks of cases reported in Florida in the last few days, when temperatures were close to 90. It's true that the virus seems to disintegrate faster in the heat... but so do humans. We spend much of our summer days indoors, with cool air conditioning, so even if feel we can SD outdoors and remove the mask, we still should use it when near others or indoors around others who might potentially be infected.